Solar collector device

ABSTRACT

A solar collector is provided in which a focussing element precisely focusses solar radiation upon a collecting region of a collecting element during all times of the day, without necessitating daily motion of the focussing element. The collecting region is constructed to be more highly absorbing of the solar radiation than any other region of the collector which might be in thermal contact with the collecting region. 
     In some embodiments, the collecting region is a self-defined portion of the collecting element upon which the solar radiation is focussed at any given time. This is achieved by utilizing a collecting element which locally converts incident solar energy to another form of energy in a non-linear manner as a function of incident solar intensity. For example, the collecting element may be fabricated from a photochromic glass which darkens when impinged upon by the focussed radiation of the sun. The collecting region is automatically self-defined by the local darkened region of the photochromic glass, which traverses the collector as the sun traverses the sky.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 203,258, filed Nov. 3, 1980,now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to devices for collecting and storingsolar radiation, and particularly to a solar collector of the type inwhich solar radiation is focussed onto a light absorbing surface.

Prior art solar collectors of the focussing type have been known formany years. Examples of such collectors, utilizing lenses, are given inU.S. Pat. No. 3,929,121, U.S. Pat. No. 3,981,295, and U.S. Pat. No.4,137,899. Each of these patents discloses a system in which a flat lensor a Fresnel lens focusses solar radiation upon a conduit containing aworking fluid. Typical systems of this type are unable to maintaincontinuous focussing of the sun's rays upon the conduit withoutcomplicated and expensive tracking equipment. See, for example, the gearand hinge arrangement illustrated in U.S. Pat. No. 3,929,121.

An attempt to solve this problem is discussed in U.S. Pat. No. 4,033,324which uses a number of spherical lenses focussing the sun's radiationonto a substrate below the lenses. This structure is less thansatisfactory, however, since the sun's rays will not be properlyfocussed upon the underlying substrate, except during a short portion ofthe day when the angle of incidence of the sun's rays properlycorresponds to the particular distance of the substrate from the centerof the spherical lens.

Another patent disclosing a panel of spherical lenses is U.S. Pat. No.2,277,311. In this reference, hemispherical lenses focus the sun's raysonto a set of opposing hemispherical lenses which diffuse the rays ontoa substrate. Although this device will collect radiation from the sunover a large portion of the day, there does not appear to be anyeffective focussing of the rays onto a collector.

A more interesting approach is taken in U.S. Pat. No. 1,093,498, inwhich focussing is accomplished using a number of spherical lenses, eachpositioned above a hemispherical collecting cup. Since the collectingcup is concentric with the spherical lens, the sun's rays will befocussed on some portion of the interior of the cup during the entireday. However, the collecting cups are fabricated from metallic radiatingcases which both absorb and re-radiate heat through the lens; the deviceis therefore inefficient as a solar collector.

In U.S. Pat. No. 3,,587,559, a similar configuration is disclosed inwhich the interior of the collecting cup is lined with a carbonimpregnated cloth to increase the absorption of solar radiation thereon.Again, however, the net efficiency on the device is not increased sincethe collector will re-radiate from all portions of the darkened cup backthrough the spherical lens.

SUMMARY OF THE INVENTION

In accordance with the illustrated preferred embodiments, the presentinvention provides a solar collector including a focussing element whichprecisely focusses solar radiation upon a collecting region of acollecting element during all times of the day, without necessitatingdaily motion of the focussing element. The collecting region isconstructd to be more highly absorbing of the solar radiation than anyother regions of the collector.

In some preferred embodiments, the focussing element is a sphericallens, while the collecting element is an arc-shaped conduit concentricwith the spherical lens and containing a working fluid.

In other embodiments, the focussing element is a hollow reflectinghemisphere, reflecting the sun's rays onto different portions of asemi-circular collecting element during the day.

In some embodiments, the collecting element may be a collecting cuphaving a darkened striplike region in the interior of the cup, or thecollecting region may be a thermoelectric of thermoionic converter.

In accordance with yet other aspects of the invention, the collectingregion is a self-defined portion of the collecting element. For example,the collecting element may be fabricated from a material havingnon-linear absorbing properties as a function of the intensity ofradiation incident thereon. Such a material is, e.g., a photochromicglass, which darkens when impinged upon by the focussed radiation of thesun. In embodiments exploiting this aspect of the invention, thecollecting region is automatically self-defined by the local darkenedregion of the photochromic glass, which traverses the collector as thesun traverses the sky. Alternatively, the non-linear conversion of solarenergy to electrical energy as a function of incident solar flux may beexploited in accordance with the invention by providing a collectingelement which includes an array of solar cells positioned so that thefocussing element focusses upon different cells at different times ofday.

Further embodiments exploit heat transfer properties which arenon-linear functions of temperature (and hence of incident solarintensity) to produce an effective heat transfer to and from the workingfluid of the collector only in the local region upon which the sun'srays are focussed. For example, the collecting element may be fabricatedof a semiconductor material whose thermal conductivity is a non-linearfunction of temperature. Alternatively, an array of uni-directional heatpipes subjected to the focussed radiation of the sun may serve as thecollecting element. In yet another such embodiment, the collectingelement may include a flexible heat conducting strip having a moving"dimple" which follows the focussed solar radiation to provide heattransfer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a spherical lens focussing the sun's rays upon a spoton a focussing circle.

FIG. 2 shows a portion of a solar collector in which a conduit ispositioned along the focussing circle.

FIGS. 3A and 3B illustrate a portion of a solar collector in which alens focusses the sun's radiation along a collecting strip lining theinterior of the collecting cup.

FIG. 4 shows a cross-section of several elements of a solar collectorshown in FIG. 3.

FIGS. 5A and 5B illustrate two views of a collecting cup in which acollecting strip is thermally insulated from the remainder of the cup.

FIG. 6 illustrates a top view of a collecting cup in which a lineararray of solar cells lines a portion of a collecting cup.

FIG. 7 shows a cross-sectional view of the collecting cup of FIG. 6 usedin conjunction with spherical lens.

FIG. 8 shows a portion of a solar collector in which a spherical lensfocusses the sun's radiation upon a thermoelectric or thermoionicconverter.

FIG. 9 illustrates a reflecting mirror focussing the sun's radiationupon a conduit positioned upon the focussing circle of the mirror.

FIG. 10 shows a solar collector utilizing two types of photochromicglass.

FIGS. 11A and 11B schematically illustrate the use of a unidirectionalheat pipe in which heat may be transferred in one direction only.

FIG. 12 illustrates a solar collector in which an array ofunidirectional heat pipes is subjected to the focussed radiation of thesun.

FIG. 13 shows a solar collector in which the focussed solar radiationinduces dimpling of a mechanical element resulting in local heattransfer.

DESCRIPTION OF THE INVENTION

To facilitate discussion of the various embodiments of the invention,the same label will be used to designate corresponding elements in thevarious figures. In FIG. 1, there is shown a spherical lens 11, forexample, of glass or plastic. Such a spherical lens has the property offocussing parallel rays of light from the sun, such as thosecollectively labeled 13, onto a focal spot 15. As the sun moves acrossthe sky during the day as indicated by the arrow 17, the focal spot 15will move oppositely around a focal arc 19 (concentric with sphere 11),as indicated by the arrow 21. The theoretical radius "r" of curve 19 isgiven by the equation

    r=R+(2-n)R/(2(n-1))

where R is the radius of focussing sphere 11 and n is the index ofrefraction of the focussing sphere, assuming that the medium betweensphere 11 and curve 19 has an index of refraction of 1. For differentchoices of media, the governing equation is different, but may be foundin the optics literature. For the case of a heavy flint glass sphere, nequals 1.8 and r equals 1.125R. Of course, in practical structures rwill vary slightly from this theoretical value, due to abberation.

In FIG. 2, a tubular conducting element 23 is physically positioned nearsphere 11 along the arc designated 19 in FIG. 1. A working fluid such aswater or oil enters conduit 23 through one opening as indicated by arrow25, passes through the semi-circular conduit and exits at anotheropening as indicated by arrow 27. The absorbing surface 29 of conduit 23is made to be highly absorbing of the radiation focussed on the conduit.For example, surface 29 may be covered with a black material such as"carbon black". As the sun traverses the sky, the concentrated solarradiation will at all times be focussed on some portion of conduit 23,thereby providing a highly efficient means for heating the working fluidpassing therethrough. The heated fluid may then be stored and/oremployed to produce energy in any of a number of ways commonly known tothose skilled in the art. It will be recognized that the system shown inFIG. 2, in which the darkened surface 29 of conduit 23 is positionedalong focussing arc 19, constitutes a very efficient way of utilizingradiation from the sun, since at all times during the day the solarradiation is focussed onto some portion of the darkened inner surface 29of conduit 23.

Another preferred embodiment of a solar collector is illustrated, inpart, in FIGS. 3A and 3B in which spherical lens 11 is used to focus thesun's rays on a region of a collecting cup 31. The collecting cup issubstantially of hemispherical shape having a radius equal to the radiusof focussing arc 19 defined above in connection with FIG. 1. Astrip-like collecting region 33 may be defined along the interior ofcollecting cup 31 so that by suitably locating collecting cup 31 withrespect to lens 11, strip-like region 33 can be positioned alongfocussing arc 19 (of FIG. 1). In FIG. 3B, there is shown a top view ofcollecting cup 31 including such a strip-like region 33 of a lightabsorbing material such as carbon black. The remaining regions ofcollecting cup 31 on either side of strip 33 are collectively labeled 35and are preferably of a reflecting material such as polished aluminum.The shape and width of collecting strip 33 are selected to insure thatthe sun's rays will be properly focussed on the strip at all timesduring the year. More particularly, if seasonal adjustment of thecollector is not desired, the collection strip must be wide enough sothat the focussed solar rays always fall on the collection strip. Thesun's arc across the sky goes through its 47° seasonal change inlatitude during a yearly cycle. Defining "W" as the width of thecollecting strip, it is easy to show that

    W=rθ.sub.annual

where r is the focal length of the sphere and θ_(annual) is the seasonalangular change of the sun's arc (0.82 radians).

The embodiments shown in FIGS. 2, 3A, and 3B illustrate an importantaspect of the invention; namely, that there is included in eachstructure a "collecting region" of the device on some portion of whichthe rays are focussed at all times as the sun traverses the sky. Thiscollecting region is blackened to provide absorption of the sun's rays,while adjacent regions of the solar collector are made reflective. Thistends to maximize the efficiency of the collector, since heat which istransferred to the adjacent non-blackened portions of the collectingelement will not be rapidly radiated away, as would be the case if theadjacent regions were also blackened. This is in contradistinction tothe solar collector structures shown in U.S. Pat. No. 1,093,498 (Thring)and U.S. Pat. No. 3,587,559 (Nonaka), discussed in the Background of thepresent specification. In the patent to Thring, the collecting cups aresaid to be "radiating casings" which will be inefficient collectors,since most of the rays of the sun will be radiated from the collector.As a partial solution to said problem, Nonaka's device includes similarcollecting cups which appear to be entirely lined with a carbon blackcloth. However, as can be seen from the positioning of the water pipesin Nonaka, heating of the water is dependent upon a general transfer ofheat from the entire blackened cup to adjacent elements of thecollector. Nonaka has therefore failed to exploit the fact that thesun's rays will be focussed only on a small strip-like portion of thecollecting cup during the day. As a consequence, the sun's rays will beabsorbed only along a strip-like region of the Nonaka collecting cup,but will be re-radiated back through the lens from all portions of thedarkened collecting cup. The device will therefore be highly ineffecientrelative to a device in accordance with the present invention, in whichonly the strip-like region of the cup is darkened.

To emphasize this distinction of the presently disclosed structure overthose of the prior art, the term "collecting region" will be used tomean the region of the collecting element upon which the sun is focussedduring the day, and which is configured to be more highly absorbing ofthe sun's radiation than regions which are not subject to the focussedradiation of the sun's rays, especially regions which are in thermalcontact with the collecting region. Examples of such collecting regionsare the darkened strip 29 of FIG. 2 and the darkened strip-like regionof FIGS. 3A and 3B.

The concentrating power of a device configured as described above may bereadily estimated as follows: let the effective area of the collectingregion be called "A_(C) ", so that

    A.sub.C =2πr.sup.2 (θ/2-θ.sup.2 /4)

The area of the circular collection aperture of the lens "A_(A) " is

    A.sub.A =πr.sup.2

where R is the radius of the lens. Therefore, the effectiveconcentrating power "C" of the collector will be

    C=A.sub.A /A.sub.C =(R.sup.2 /2r.sup.2)/(θ/2-θ.sup.2 /4)

For the case of a heavy flint glass sphere where r equals 1.125R, andrecalling that θ_(annual) equals 0.82 radian,

    C≃1.6.

Therefore, it is seen that the spherical lens focusing embodiment willhave a concentrating power of approximately 2 suns.

FIG. 4 illustrates a portion of a solar collector utilizing focussingspheres 11 and collecting cups 31 to form an array of solar collectors.While only two elements of the array are illustrated in FIG. 4, it willbe evident to those skilled in the art that a two-dimensional array maybe formed utilizing the basic structure shown in FIG. 4. In particular,the use of collecting cups provides a convenient mechanism forpositioning each focussing sphere relative to an associated collectingstrip. Thus, in FIG. 4, a focussing sphere 11 is attached to a supportbracket 37 which is attached to a collecting cup 31. A conduit 23 isaffixed to the bottom of cup 31, adjacent to, and in thermal contactwith, collecting strip 33. Conduit 23 is thermally insulated from therest of the structure by means of insulating blocks 39. In an array ofcollectors, collecting conduit 23 will extend continuously from onecollecting cup to another, so that the working fluid entering at port 25and exiting at port 27 will pass beneath the collecting strips of eachcup, being heated by the concentrated rays of the sun from each lensduring its passage through the collecting tube. The use of collectingcups provides the additional advantage that a vacuum region 41 may beformed between lens 11 and collecting cup 31. This is advantageous inthat it reduces thermal leakage by preventing the convective conductionof heat from the cup to the lens.

In FIGS. 5A and 5B, the collecting region 33 of a collecting cup 31 isthermally insulated from adjacent regions 35 of the collecting region33. Insulating strips 43 are preferably fabricated from a ceramic orglass having a high thermal resistance and prevent or substantiallyreduce the transfer of heat collected by collecting region 33 to theremainder of the collecting cup. This configuration further minimizesre-radiation of collected heat, insuring maximum efficiency of thetransfer of the collected heat to the working fluid in a conduit 23underlying collecting region 33.

In the embodiment shown in FIG. 6, a collecting cup 31 includes astrip-like collecting region 33 along which is located a linear array ofsolar cells, for direct conversion of the focussed solar radiation intoelectricity. The solar cells may be of various types commonly available,such as a gallium arsenide cell, or a silicon cell. A particularadvantage of this embodiment of the invention is that many solar cellsbecome more efficient at increasing light intensities, such as theintensity resulting from the solar focussing according to the invention.An additional advantage of this embodiment is illustrated in FIG. 7, inwhich the linear array of solar cells 45 is used both to generate directelectrical power, and also to heat a working fluid. This is possiblebecause the focussed rays of the sun will heat the solar cells to theiroperating temperature of about 150° C., which heat may be transferred toa working fluid in a conduit 23 positioned under the linear array ofsolar cells. Thus, in this embodiment both the heated working fluid andthe voltage produced by the solar cells (appearing at terminals 47) areavailable as energy sources.

In yet another embodiment of the invention shown in FIG. 8, a darkabsorptive coating 49 is placed along a strip-like collecting region 33of a collecting cup having a metallic inner surface 51 functioning asthe anode of a thermoelectric or thermoionic converter. The bottomsurface 53 is cooled by a coolant flowing in a conduit 54 and functionsas a cathode of the converter. In operation, a temperature differentialbetween anode and cathode will be established, producing an electricalvoltage at output terminal 55.

In FIG. 9, there is illustrated an embodiment of the invention in whicha hollow hemisphere 57 having a highly reflective inner surface 59, forexample, of highly polished aluminum, acts as a concave spherical mirrorfocussing the sun's rays 61 onto a spot 63 on the underside 33 of aconduit 23 through which flows a working fluid. Conduit 23 issemi-circular in shape, having a center co-incident with the center ofthe reflecting sphere, so that as the sun's rays 61 impinge onreflecting sphere 57 at different times of the day, focussing spot 63will traverse collecting region 33. As in other embodiments of theinvention, the underside of conduit 23 is preferably made highlyabsorptive of the sun's radiation, for example, by coating with adarkened substance such as carbon black.

The concentrating power of this mirror embodiment may be calculatedusing the formulas earlier defined for the lens embodiment, while notingthat for the case of a spherical mirror, r=1/2R. Thus,

    C≃8.

Thus, the ideal concentrating power of this embodiment is about 18 suns(compared to 1.8 suns for the lens embodiments). However, for otherreasons, such as effects of internal reflections, correction losses,collector shadowing by support elements, etc., the practicalconcentrating power of the spherical lens embodiment will be somewherewithin the range of 1.25 to 2.5 suns and that of the mirror embodimentwithin the range of 5 to 10 suns.

These concentrating powers may be increased in direct proportion to thenumber of times during a year it would be permissible to move thecollector. For example, if the collector can be repositioned once everysix months, the collector strip area may be reduced by approximately afactor of two, thereby increasing the concentrating power of thespherical lens embodiment to 2 to 4 suns and that of the mirrorembodiment to 8 to 13 suns.

The choice of which embodiment to employ depends upon the desiredapplication. For example, in especially cloudy regions, theeffectiveness of the collector under diffused light conditions becomesimportant. In such an application, the spherical lens embodiment wouldbe preferred since its concentrating power would drop to approximately1/2 sun, where as that of the mirror embodiment would drop toapproximately 1/10 sun.

In accordance with yet another aspect of the invention, the collectingelement may be fabricated from a material having non-linear absorbingproperties as a function of the intensity of radiation incident thereon;such a material is the photochromic glass presently used in themanufacture of certain types of sun glasses, available from the CorningGlass Co., which has the property of becoming darker when impinged uponby sunlight. In preferred embodiments, the collecting cup 31 of FIG. 3may be fabricated in its entirety, or alternatively only along thecollecting region 33 from such a photochromic glass. When the rays ofthe sun are converged by lens 11 on a small spot lying along collectingregion 33, that spot will become much darker than the remainder of thecollecting cup and thereby absorb the sun's radiation at a much higherrate than the remainder of the collecting cup. As the sun traverses thesky, different portions of collecting region 33 will become dark. Thus,the nonlinearity in absorption of the sun's rays by the photochromicglass is exploited to provide the collecting region of the device. Asimilar structure may be achieved in the context of FIG. 2, in whichcase the top surface of conduit 23 may be fabricated from thephotochromic glass as well as in the contexts of FIGS. 5 or 9. Theembodiments of the invention utilizing such a non-linear absorbingmaterial are even more efficient solar collectors than those earlierdescribed, since at any given time only a small spot of the collectingregion is, in fact, more highly absorbing (and, hence, emitting) thanthe remainder of the collector. It should be noted that the use of sucha non-linear absorbing material would also be advantageous in many othersolar collectors in which solar radiation is focussed on a collectingelement, even where the structure as a whole did not provide theadditional advantages of the various embodiments disclosed herein.

As an alternative to the non-linear absorption of the above describedphotochromic glass, other properties which are non-linear as a functionof solar flux or temperature may also be exploited in connection withthe invention. For example, certain materials, e.g. most glasses andsome semiconductors, become good conductors of heat at elevatedtemperatures, whereas they are poor heat conductors at lowertemperatures. Thus, the collecting cup 31 (or the collecting region 33)may be lined with such a material, then only the focal spot will becomehot enough to lower the thermal resistance to the point where the heatwould readily be transferred to and from the working fluid of thecollector. The remaining regions of the collector would be at a lowertemperature, and hence could not readily transfer any heat to be lost asre-radiation from the collector. Note that the photovoltaic arraypreviously discussed displays a related property, since the conversionof solar flux to electricity is more efficient at the higher solarconcentration incident on the particular cell which is illuminated atany given time of day.

Another embodiment utilizing photochromic glass or the like is shown inFIG. 10. In this embodiment, a secondary layer of photochromic glass 65is positioned between the focussing element 11 and a primary layer 67 ofphotochromic glass. However, unlike primary glass layer 67 which darkenswith increasing light intensities, second layer 65 is nominally dark inlow light intensities, while becoming transparent with increasing lightintensities. The properties of both layers are selected such that indiffuse light conditions (when the light intensity is too low to causethe primary layer to darken), the secondary layer remains dark, andtherefore absorbs the solar radiation. The concentrating power of thecollector would be "1" under such conditions, as opposed to virtuallyzero in the single layer embodiment under similar conditions. Since thelight collection takes place in two different regions determined bydirect or diffuse solar lighting conditions, two different heattransport systems are preferred for optional operation. The first, orlow temperature system, would use a transparent collecting shell 69 indirect thermal contact with secondary photochromic layer 65. Inside tube69 circulates a transparent heat transferring fluid 71 (gas or liquid).The second, or high temperature system, uses a collecting tube 23 andfluid as described above in connection with earlier embodiments. Theprimary and secondary photochromic glass regions are separated from eachother and from lens 11 by vacuum regions 71 and 73, respectively. Foroptimal performance, incoming solar rays 75 should be focussed uponphotochromic layer 67 by the combination of lens 11 and the effective"lens" formed by photochromic glass 65, tube 69, and fluid 71. Thisembodiment of the invention may be employed in circumstances requiringthat the solar energy system function at least as well as a flat platecollector under diffuse light conditions.

Yet other embodiments are available in which the collecting element isfabricated from a device or devices having non-linear heat transferproperties. One such device is known as a heat pipe and transfers heatreadily and efficiently in one direction only. FIGS. 11A and 11B showsuch a device in which a fluid 77 is located at a collector end 79 of atube 81. When the fluid is heated to its boiling point, it evaporates,travels to a transfer end 83 of pipe 81, and there condenses; heat hastherefore been transferred from collector end 79 to transfer end 83.However, if heat is applied to end 83, no heat will be transferred inthe reverse direction, since there is no fluid gathered at end 83 to beevaporated; this condition is shown in FIG. 11B.

Such a heat pipe may be utilized in the present invention as shown inFIG. 11, where an array of heat pipes 85 are positioned with theirtransfer ends adjacent a collecting tube 23. A hemispherical mirror 57focusses the sun's rays onto a small region of the heat pipe array. Thecollector end of each heat pipe is thermally insulated from its neighborin the array so that only the illuminated pipe (or pipes) becomes hotenough to boil the working fluid contained therein. The resulting vaporcondenses on the transfer end 83 of the heat pipe, thereby giving upheat to the working fluid of the collector in conduit 23. The condensedfluid then flows back down to the collector end 79 to repeat theprocess. Now, even though the transfer ends of all pipes are in mutualthermal contact (through conduit 23), heat cannot flow from the transferends of the non-activated pipes to their collector ends. Therefore, thecollector ends of the non-illuminated heat pipes cannot be heated andhence cannot radiate away the collected solar energy. Thus, without anymoving parts, the working fluid in conduit 23 can be heated to a veryhigh temperature as efficiently as a conventional focussing solarcollector that mechanically tracks the sun.

A collector according to aspects of the invention, having non-linearheat transfer properties may also be constructed using mechanical meansas shown in FIG. 13. Again, a hemispherical mirror 57 focusses the sun'slight onto different local spots 87 at different times during the day.In this embodiment two strips of metal 89 and 91 are held apart by athermally insulating material 93. Strip 89 is in contact with theworking fluid in conduit 23 and is a good thermal conductor. Strip 91 isconstructed of a flexible material that is a poor thermal conductor suchas a very thin strip of stainless steel or nitinol, a metal withmechanical memory. The side of the strip 91 facing mirror 57 is coatedwith an absorptive coating. The space between the two strips should be arelatively good vacuum to prevent heat conduction across the gap. Strip91 is mechanically stressed such that when a localized region 87 isheated, the strip "dimples" inward until it touches strip 89. At thepoint 87 where the strips touch, a thermal bridge is formed, increasingdramatically the thermal conductivity from the heated strip 91 to theopposite strip 89 in thermal contact with the working fluid in conduit23.

The zone of local heating will therefore follow the focal spot 87 as thesun moves across the sky. By construction, the thermal gradients willmove the "dimple" in such a manner that it will always be located in thehotest zone, i.e., the focus of the collector. In this way, the regionof highest thermal conductivity would track the focal zone of the solarradiation.

Because of the interspatial vacuum, very little heat will flow back fromthe working fluid to regions of outer strip 91 other than the local spot87. To facilitate heat transfer at the point of contact, as well as toprovide some lubrication, the interior sides of the strips could becoated with some material such as mercury.

I claim:
 1. A device for collecting solar radiation comprising:a conduitcontaining a fluid; an array of unidirectional heat pipes, each pipehaving a collecting end upon which solar radiation is to be focussed anda transfer end in thermal contact with said conduit, with each pipebeing closed under normal operating conditions to preclude entrythereinto of fluid from said conduit, said array having no fluid pathconnecting any one pipe to any other pipe in said array to preventtransfer of fluid from one pipe to another pipe; and means for directingsolar radiation onto different heat pipes of said array during differenttimes of the day.
 2. A device as in claim 1 wherein said means comprisesfocussing means positioned in spatial relationship with said array forfocussing solar radiation on different heat pipes of said array atdifferent times of day.
 3. A device as in claim 2 wherein said focussingmeans and said array are configured so that solar radiation is focussedon different heat pipes of said array at different times of day withoutnecessitating motion during said day of said focussing means or saidarray of heat pipes.
 4. A device as in claim 3 wherein:said focussingmeans comprises a hemispheric mirror.